A number of medical ailments are treated or treatable and/or diagnosed through the application of a magnetic field to an afflicted portion of a patient's body. Neurons and muscle cells are a form of biological circuitry that carry electrical signals and respond to electromagnetic stimuli. When an ordinary conductive wire loop is passed through a magnetic field or is in the presence of a changing magnetic field, an electric current is induced in the wire.
The same principle holds true for conductive biological tissue. When a changing magnetic field is applied to a portion of the body, neurons may be depolarized and stimulated. Muscles associated with the stimulated neurons can contract as though the neurons were firing by normal causes.
A nerve cell or neuron can be stimulated in a number of ways, including transcutaneously via transcranial magnetic stimulation (TMS), for example. TMS uses a rapidly changing magnetic field to induce a current on a nerve cell, without having to cut or penetrate the skin. The nerve is said to “fire” when a membrane potential within the nerve rises with respect to its normal negative ambient level of approximately −90 millivolts, depending on the type of nerve and local pH of the surrounding tissue.
The use of magnetic stimulation is very effective in rehabilitating injured or paralyzed muscle groups. Apart from stimulation of large muscle groups such as the thigh or the abdomen, experimentation has been performed in cardiac stimulation as well. In this context, magnetic stimulation of the heart may prove to be superior to CPR or electrical stimulation, because both of those methods undesirably apply gross stimulation to the entire heart all at once.
Another area in which magnetic stimulation is proving effective is treatment of the spine. The spinal cord is difficult to access directly because vertebrae surround it. Magnetic stimulation may be used to block the transmission of pain via nerves in the back, e.g., those responsible for lower back pain.
Magnetic stimulation also has proven effective in stimulating regions of the brain, which is composed predominantly of neurological tissue. One area of particular interest is the treatment of depression. It is believed that more than 28 million people in the United States alone suffer from some type of neuropsychiatric disorder. These include conditions such as depression, schizophrenia, mania, obsessive-compulsive disorder, panic disorders, and others. Depression is the “common cold” of psychiatric disorders, believed to affect 19 million people in the United States and possibly 340 million people worldwide.
Modem medicine offers depression patients a number of treatment options, including several classes of anti-depressant medications (e.g., SSRI's, MAOI's and tricyclics), lithium, and electroconvulsive therapy (ECT). Yet many patients remain without satisfactory relief from the symptoms of depression. To date, ECT remains an effective therapy for resistant depression; however, many patients will not undergo the procedure because of its severe side effects.
Recently, repetitive transcranial magnetic stimulation (rTMS) has been shown to have significant anti-depressant effects for patients that do not respond to the traditional methods. The principle behind rTMS is to apply a subconvulsive stimulation to the prefrontal cortex in a repetitive manner, causing a depolarization of cortical neuron membranes. The membranes are depolarized by the induction of small electric fields in excess of 1 V/cm that are the result of a rapidly changing magnetic field applied non-invasively.
Creation of the magnetic field has been varied. Certain techniques describe the use of a coil to create the necessary magnetic field. Other techniques contemplate the use of a high saturation level magnetic core material, like vanadium permendur. Use of the magnetic core material, as compared to the coil or so-called “air” core solution, has been shown to increase the efficiency of the TMS process. For example, as discussed with reference to U.S. Pat. No. 5,725,471, using a magnetic core instead of just a coil increases the efficiency of the TMS process by creating a larger, more focused magnetic field with the same or lesser input power requirements.
This advance has allowed a more cost effective solution that uses existing 120 volt power without complicated and a costly power supplies. Also, because of the need for the same or lesser power inputs, the magnetic core significantly reduces the undesirable heating that was associated with the coil solution and created a safety risk for patients. For example, magnetic core devices in comparison to coil-only devices reduce the magnetic reluctance path by a factor of two. This reluctance reduction translates into a reduction of required current to generate the same magnetic field by the same factor, and thus provides a fourfold reduction in required power.
The ferromagnetic core alternatives typically are fabricated by laminating layers of silicon steel or similar ferromagnetic metal together to form the core structure. The layers may be constructed by stacking cut-out shapes or by winding a ribbon of material onto a mandrel followed by further machining and processing to attain the desired core geometry.
While solutions fabricated using these ferromagnetic cores offered a marked improvement over their coil-only counterparts, the ferromagnetic cores also suffer from certain complexities in their construction and limitations in their geometry. Specifically, the stacked layer construction method does not provide optimal alignment of the metal crystal structure with the magnetic flux lines and also requires a controlled lamination process to guarantee minimal eddy current losses. The wound ribbon construction method typically results in a core with arc-shaped or C-shaped structure having a certain radius and span. The dimensions and geometry of these ferromagnetic cores are selected to ensure desired depth of penetration, magnetic field shape and appropriate magnetic field magnitude at certain locations within the patient's anatomy.
The ferromagnetic core's construction method involves a complex and meticulous construction process that increased both the complexity and cost of the core. For example, because ferromagnetic material is electrically conductive, eddy currents are established in the material when it is exposed to a rapidly varying magnetic field. These eddy currents not only heat the core material through resistive heating, but they also produce an opposing magnetic field that diminishes the primary magnetic field. To prevent these losses the eddy current pathways are broken by fabricating the core from very thin layers or sheets of ferromagnetic material that are electrically isolated from each other.
The sheets typically are individually varnished or otherwise coated to provide insulation between the sheets, thus preventing current from circulating between sheets and resulting in reduced eddy current losses. Also, the sheets are oriented parallel to the magnetic field to assure low reluctance.
The wound core fabrication process begins by winding a long thin ribbon of saturable ferromagnetic material, such as vanadium permendur or silicon steel, on a mandrel to create the desired radius, thickness and depth of the core. Each side of the ribbon typically is coated with a thin insulative coating to electrically isolate it. Once the ribbon has been wound on the mandrel to the desired dimensions, it is removed from the mandrel and dipped in epoxy to fix its position. Once the epoxy has cured, a sector of the toroidal core is cut with a band saw and removed, thus forming the desired arc-shape. Because the cutting process may reduce the electrical isolation of adjacent laminations, each cut is finely ground so that it is smooth, and then a deep acid etch is performed. The deep etch is performed by dipping each of the cut ends in an acid bath. This removes any ferromagnetic material that may be shorting the laminations. Following the deep etch, the faces are coated to prevent oxidation and to maintain the shape and structural integrity of the core. The manufacturing process of cutting, coating, aligning, attaching and laminating the layers makes for a complex and costly manufacturing process. Also, these considerations make it difficult to change or customize the shape of the core structure.
Winding a coil of insulated wire around the ferromagnetic core to deliver the current needed to create the magnetic field also is a complex and detailed process. A typical inductance for a core of this type is about 15-20 microHenries. Each pass of the winding around the core must be made at precise intervals on the core structure. In the simplest configuration, each core has only one winding, although typically the core may be wound multiple times.
While the present ferromagnetic core shape and composition work well, and certainly better than the coil-only approach, it should be appreciated that other core compositions and core shapes may work equally well under other circumstances.